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Understanding tick-borne diseases

Thursday, May 17, 2012

A Primer for Hudson Valley Residents and Healthcare Workers

Millbrook, NY – The Hudson Valley has the unfortunate distinction of being the global epicenter of tick-borne disease. And the situation is getting worse. First it was Lyme disease. Then we were invaded by a series of tongue-tying maladies, among them anaplasmosis, ehrlichiosis, and babesiosis. All of these diseases are transmitted to us through tick bites. But which ticks are responsible? And what can be done to minimize our risk?

Five infectious disease experts put together this advisory to clarify confusion on the tick-borne diseases we face, including ticks responsible for their spread and tests that healthcare providers can perform to diagnose them.

nymph blacklegged tick

The Diseases

Lyme disease is caused by a corkscrew-shaped bacterium called Borrelia burgdorferi. A circular (bull’s eye) rash is common at the site of the bite. In 20-30% of cases, no rash occurs. Anaplasmosis, or human granulocytic anaplasmosis, is caused by a bacterium called Anaplasma phagocytophilum, which attacks a type of white blood cell (granulocytes). This bacterium was initially misidentified, and the disease was erroneously called human granulocytic ehrlichosis. There is no such disease; however, there is another disease called human monocytic ehrlichiosis. This is caused by the bacterium Ehrlichia chafeensis, which attacks a different type of white blood cell (monocytes). Babesiosis is a more recent invader, caused by a protozoan parasite (related to the malaria parasite) called Babesia microti. And now several cases of encephalitis (brain swelling) caused by the tick-borne Powassan virus have surfaced in our area.

The Ticks

In our region,the blacklegged tick is exclusively responsible for spreading Lyme disease, anaplasmosis, and babesiosis. These animals are commonly referred to as deer ticks. Scientists prefer using the correct name –the blacklegged tick – because it counteracts misperceptions that their numbers are tied to deer. White-footed mouse populations play a much stronger role in regulating blacklegged ticks.

A more southerly tick species, the lone star tick, appears to be moving northward and invading the Hudson Valley, although they’re not yet widespread. Lone star ticks carry monocytic ehrlichiosis. In our area we also have dog ticks, which are relatives of the wood tick (which is not found here). Dog ticks spread Rocky Mountain spotted fever, which could become more common in our region.

All of these ticks take three blood meals during their lifetime – as a larva, as a nymph, and as an adult. Most larval ticks are born pathogen-free; they acquire disease agents when they feed on infected wildlife hosts. Once infected, ticks pass the disease on when they bite later as a nymph or adult. About 30% of blacklegged tick nymphs and 60-70% of adults are infected with Lyme disease bacteria; for anaplasmosis and babesiosis, infection is much lower. Ticks can be infected with multiple disease agents. For patients that fall victim to these ticks, symptoms can be particularly severe.

Diagnostic Tests

Blood work can help diagnose tick-borne diseases. Labs look for antibodies that reveal that your body has been exposed to a disease agent. It can take a few weeks to produce antibodies, so blood drawn days or even weeks after a tick bite will often test negative—even if the pathogen is present. Because the standard blood test for Lyme disease is imperfect, many doctors diagnose based on tick bite exposure and symptoms (bull’s eye rash, flu-like signs), using blood work as a final confirmation. If you find a tick attached to you or a loved one, save it and have it identified (Cornell Cooperative Extension in Millbrook is able to identify ticks). Should you get sick, insight into the species will help medical staff manage treatment. Having the tick tested is not recommended, because a positive tick might not have transmitted any pathogens. And a negative tick can mask the possibility that some other undetected tick transmitted disease.

Treatment

With the exception of babesiosis, most tick-borne diseases in our region can be cured with antibiotics. When detected early, Lyme disease is typically treated with oral doxycycline or amoxicillin. Ehrlichiosis and anaplasmosis infections also respond well to oral doxycycline. Most cases of babesiosis resolve without treatment. Patients with long-standing babesiosis infections may require a combination of two anti-parasite drugs (atovaquone and azithromycin).

Prevention

When you are active outdoors in wooded or shrubby areas any time the temperature is about 40 degrees F or warmer, be vigilant about performing tick checks. Stay on hiking trails and wear light colored clothing; this helps in the detection of ticks, which are dark-colored. If you are going to be in tick habitat, consider applying a product with 20% DEET on your pants and shoes.

The Future

Keeping a handle on tick-borne diseases is a challenge for ecologists, public health professionals, health care providers, and the public. Here in Dutchess County, ecologists are constantly monitoring the abundance of ticks and the pathogens they carry. This includes studies of what controls tick populations and how this information might be used to protect public health. Public health professionals are working with doctors to inform them of new developments in diagnosis and treatment of tick-borne diseases. They also educate the public about minimizing exposure risk. Other researchers throughout the country are working on better diagnostic tests, treatments, and vaccines.

In the meantime, knowledge about the ticks and the pathogens they carry can help us stay healthy.

Christen Herzog, Public Health Advisor, Dutchess County Department of Health

Michael C. Caldwell, MD, MPH, Dutchess County

The Cary Institute of Ecosystem Studies is a private, not-for-profit environmental research and education organization in Millbrook, N.Y. For more than twenty-five years, Cary Institute scientists have been investigating the complex interactions that govern the natural world. Their objective findings lead to more effective policy decisions and increased environmental literacy. Focal areas include air and water pollution, climate change, invasive species, and the ecological dimensions of infectious disease.